Concrete, a cementitious compound, is known as one of the most versatile and most widely produced building materials in the world. Concrete is a gel, being a particulate strengthened ceramic-matrix composite material comprised of mortar and aggregate. Mortar is typically comprised of a hydraulic cement such as Portland cement clinker, together with water and sand. Thus, in concrete, the sand and stone become dispersed particles in a multi-phase matrix of Portland cement paste, which is comprised of the cement clinker and water.
Portland cement clinker is relatively anhydrous such that it tends to have an affinity to water. Thus, when water is added to the clinker, the clinker reacts with the water to form the cement paste. The chemical products of this reaction, which are found in the cement paste, provide strength and cohesiveness to the concrete. As time passes, the cement hydrates further, which produces more reaction products. Therefore, the strength and cohesiveness of the cementitious compound or cement typically improve with the passage of time.
It is known that although concrete is relatively strong in compression, it tends to be somewhat weaker in tension, and thus in flexion. Although increasing the amount of cement clinker or supplementary cementitious materials, as discussed below, in the concrete may improve the tensile and flexural strength, along with the compressive strength, of the concrete, this tends to be somewhat cost prohibitive and may be problematic to the maintenance of a desirable water/cement ratio. For instance, it is well known that in conventional concretes, the flexural strength tends to be about 15% or 1/7 of the compressive strength. Therefore, for every desired strength unit increase in the flexural strength (such as 1 Mpa), the compressive strength must be increased by 7 strength units (7 Mpa).
As a result, the industry has sought other approaches to reinforcing concrete in order to improve its strength, and in particular its tensile and flexural strength. One approach is to incorporate steel reinforcing bars into the concrete to provide the required tensile and flexural strength. The concrete provides both a highly alkaline environment which is compatible with the steel and a physical barrier to protect the steel from exposure to the environment.
In more recent years, the industry has also developed a number of additives which are added to, or incorporated into, the concrete in order to improve various properties of the concrete, including its tensile, flexural and compressive strength. Strength improving additives are solids which tend to be either inert, such that their mere presence in the concrete adds to its strength, or reactive, such that the additive is involved in the formation of the reaction products which give concrete its strength and other desirable properties.
Waste products, such as fly ash, blast furnace slag and condensed silica fume, are one such group of reactive strength improving additives which are often added to concrete. The addition of such waste products tends to reduce the amount of cement required in the mix to produce the concrete, which may result in a cost benefit. Further, these waste products, which otherwise have few uses, become useful and thus, there is a benefit to the environment as the waste products are incorporated into a useful produce rather than requiring disposal. However, perhaps most significantly, the properties of the concrete, and in particular its strength, tend to be significantly improved.
However, many of the additives, including the waste products noted above, are silicates which tend to be hydrophilic to varying degrees. As a result, use of these additives often requires an increase in the water content of the concrete. Further, the waste products are typically fine or very fine powders which also increases the water requirement of the concrete due to their large surface areas. Therefore, when these additives are incorporated into the concrete, a greater quantity of water must typically be added, which results in an increase in the water/cement ratio of the concrete. A minimum water/cement ratio, or a preferred range of water/cement ratios, is required to allow for proper hydration of the cement and to permit the concrete to be workable. However, preferably, any excess water is minimized as too high a water/cement ratio will decrease the strength of concrete and have other adverse effects on its properties.
The water/cement ratio (the ratio of mass of water to mass of dry cement) is one of the most important parameters in determining the properties of the hardened concrete. In theory, a water/cement ratio of approximately 0.23 is needed for complete hydration of the clinker. However, the total volume of water contained in the gel pores of the concrete increases the required water content by approximately 0.19, resulting in a theoretical total minimum water/cement ratio requirement of approximately 0.42 for complete hydration. However, in practice, and due to a number of factors, additional water is often required to produce a workable mix. The water/cement ratio required to produce a workable mix must be carefully considered in light of the known adverse effects of excess water in the concrete and high water/cement ratios.
Any excess water in the mix tends to exist in the spaces between the original cement particles and between the cement and any aggregate. These spaces do not fill completely with the gel, but rather, form a network of "capillary pores" containing water. As stated, it is desirable to add enough water to the concrete to provide for a workable mix, while minimizing the capillary pores. Thus, a balancing is required between the workability of the concrete and the detrimental effect of the capillary pores on the properties of the concrete. However, due to the natural hydrophilic properties of many additives, the problems related to the presence of capillary pores, and the problems associated with achieving the "right" balance, are typically worsened as the additives tend to require the addition of more water in order to produce a workable concrete. As the percentage of fine particle additives is increased, more water is typically required to produce a workable mixture and thus, the water/cement ratio is increased.
The balancing of the workability of the concrete mix and the water/cement ratio of the concrete mix can be assisted to some degree by the use of water reducing agents and high range water reducing agents (superplasticizers), which are typically added to the concrete mix in liquid or powder form prior to the placement of the concrete. These water reducing agents cause the components of the concrete mix temporarily to repel one another, thus reducing the water demands of the concrete mix. Unfortunately, this effect is not selective, with the result that a large proportion of the water reducing agent that is added to the concrete mix is used to produce the effect in components the concrete mix, such as coarse aggregate and sand, which do not otherwise place high water demands on the mix. Since these components typically make up a large percentage by weight of the overall mix design, it can be seen that the conventional use of water reducing agents is not an efficient way of reducing the water demands of a concrete mix. This, together with the high cost of some water reducing agents (particularly superplasticizers) and the sometimes undesirable side effects (such as retardation of curing of the concrete mix) resulting from their use demonstrates the need for an alternate means for reducing the minimum required water/cement ratio to produce a workable concrete mix.
There is therefore a need in the industry for an additive for incorporation into a cementitious compound comprising Portland cement and water, such as concrete, that improves the properties of the compound, and in particular its strength, but does not attract the disadvantages, or cause the problems, typically associated with conventional additive use (such as the need for increased water/cement ratios to produce a workable compound). In other words, the additive results in improved properties of the cementitious compound upon setting but does not substantially increase the required water/cement ratio. Specifically, there is a need for an additive for incorporation into the cementitious compound that is less attracted to the water in the compound during the placement of the compound, yet is capable of participating with the compound, during the curing of the compound, following its placement. In addition, there is a need in the industry for a process for preparing such an additive and a process for preparing a cementitious compound comprised of the Portland cement, water and the additive.